Disposal of waste water sludge is a major problem in water purification plants. This is partly due to the heavy metal content of the sludge. It is difficult to find suitable places for the waste and as standards rise landfilling is becoming more and more expensive. From this perspective the idea of recycling the waste water sludge is becoming increasingly important. A complete recycling of waste water sludge would involve recycling of coagulants (iron and aluminium), part of the organic substances of the sludge, recovery of phosphorus, and separation of the heavy metals from the sludge. Until now recycling of the sludge has been realized only partially. There are no existing production methods for separating coagulant chemicals and phosphorus from the sludge.
Sludge comes from various sources of the waste water purification process i.e. from pre-precipitation, simultaneous precipitation and post-precipitation stages. One possible treatment procedure for the sludge is first to dewater it to a dry solids content of 15-25% and then to use in agriculture, compost, incinerate or transport the dewatered sludge to a dump.
Another possible procedure is to acidify the precipitation sludge to dissolve metals. Insoluble substances are removed by filtering. The dissolved metals and phosphorus in the filtrate are precipitated and a sludge, which will be called a metal sludge, is obtained. The metal sludge contains the iron and aluminium of the used coagulant and, in addition, phosphorus and heavy metals. The procedure can also be performed at an elevated temperature to improve yield and filterability i.e. the dewatering properties of the sludge. The sludge to be treated can be a pre-precipitation sludge, a simultaneous precipitation sludge, post-precipitation sludge or a mixture thereof.
One additional alternative for treating the sludge is hydrolysis where the purpose is to hydrolyse organic material of the sludge into short-chained compounds to be utilized in later stages of the waste water treatment process, especially as carbon source in the denitrification stage. During hydrolysis, the metals of the raw sludge dissolve in the hydrolysate solution. In the so-called thermal acid hydrolysis the temperature is 150-160.degree.C. and pH&lt;2 preferably 1-1.5. After the hydrolysis, the insoluble part i.e. the organic sludge is separated, the sludge containing mainly insoluble organic and partly inorganic material e.g. fibres and silicate minerals. The pH of the obtained solution is raised above the neutral level using a base so that the dissolved metals precipitate as hydroxides and phosphates. The precipitated sludge i.e. the metal sludge is then separated. The metal sludge contains iron and aluminium and also phosphorus and heavy metals.
Acidification nor hydrolysis of sludge is not commonly used in waste water purification. One reason is poor profitability. An additional problem is the metal sludge which has no use. The metal sludge contains heavy metals which makes the sludge a harmful waste for the environment.
The metal sludge can be dissolved in sulphuric acid or possibly in hydrochloric acid and the insoluble substances can be filtered. The acidic filtrate solution contains the coagulants, phosphorus and heavy metals. It cannot be recycled or utilized in any other way as such and there are no existing methods to separate the elements.
Solvent extraction i.e. liquid-liquid extraction is a well-known method for separating different elements from each other and, in principle, it could be used to separate said elements. However, there are difficulties in applying extraction to the acidified metal sludge of the above kind or to any other acidified waste water sludge. The acidic solution obtained by leaching waste water sludge with sulphuric acid not only contains dissolved metals but also insoluble fine solid particles, colloidal components, humic acids etc. These impurities comprise an undesirable organic residue (crud) which has the most unfavourable effect on extraction. It significantly retards mass transfer and phases disengagement. In disengagement of phases, after contacting the organic and aqueous phases, this substance usually collects as a separate layer between the phases. Therefore, the existence of the insoluble residue has prevented extraction methods from being exploited in the recovery of iron and aluminium from acidified sewage sludge.
Solvent extraction of metals from strongly acidic solutions is known from treatment of spent pickling bath solutions. The patent publication U.S. Pat. No. 5,051,186 presents such a method for separating iron and zinc utilizing solvent extraction with diethyl hexyl phosphate (DEHPA) as the extracting agent. Aluminium recovery by using solvent extraction with a mixture of monoethyl hexyl phosphate (MDEHPA) and DEHPA has been suggested by Cornwell and Zoltek in J. Water Pollut. Control Fed., Vol 49, p. 600-612. A process employing solvent extraction with organic extractants for the removal of iron from aqueous acidic solutions has been suggested in the patent publication EP 58148 where the objective was to recover pure acid by extracting iron ions into organic solvent. Solvent extraction used for the selective recovery of dissolved iron and aluminium can, with a proper solvent, efficiently separate iron and aluminium from heavy metals.